The invention is generally directed to an electronic timepiece with a self-charger and a storage device for storing the charge generated by the self-charger and in particular to a self-charging electronic timepiece utilizing a solar battery for an improved charge and storage system.
Electronic timepieces with self-chargers tend to supply energy to the timepiece mechanism for a long period of time without failure. As a result of the recent reduction in the cost of solar batteries, electronic timepieces incorporating solar batteries have been developed which have long useful lives. The solar battery, which performs a charging function, is considered the primary source of energy in these timepieces. However, electronic timepieces also have various storage means (secondary sources) to prevent the electronic timepiece from stopping when there is no flow of energy from the primary source, such as at night or when the timepiece is in a drawer.
Japanese patent laid open publication No. 52-67673 discloses a storage device applied to a button shaped battery which results in an extended life for a small timepiece with limited space for the battery.
Japanese patent laid open publication Nos. 51-121366 and 55-13498 disclose improved charge-storage systems which prevent the primary source from overcharging the storage device. In addition, Japanese patent laid open publication No. 58-137784 discloses an electronic timepiece which switches the source of power from a solar battery to a galvanic cell based on the magnitude of incident light.
Recently, to achieve a more reliable and longer-lived timepiece a solid electrolyte or condenser type storage device has been used. For example, Japanese patent laid open publication No. 58-176570, discloses a timepiece in which the time during which the hands are stopped due to inadequate voltage is corrected by rapid movement of the hands after charging is completed. However, the secondary sources tend to have a small capacity which requires an improvement in the charge-storage system or charge-discharge system.
In addition, the prior art electronic timepieces with a self-charging feature tend to operate unstably when the voltage of the power source applied to the electronic circuitry drops below a normal operating range. When the secondary energy source is exhausted the timepiece will not begin to operate until the energy supplied from the primary source is great enough to generate a satisfactory operating voltage level for the electronic circuitry of the electronic timepiece. This is especially a problem in situations where the primary source generates only small mounts of current, for example, in a low light situation where the primary source is a solar battery.
Reference is made to FIG. 7 wherein a functional block diagram of an electronic timepiece 700 with a solar battery 701 constructed in accordance with the prior art is depicted. The voltage across a timepiece circuit 702 and a secondary battery 703 are always at the same potential. Therefore, when the voltage across second battery 703 is 0 volts the voltage generated by exposing solar batteries 701 to light is only about 0.6 volts. A diode 704 prevents voltage from flowing back into solar battery 701 from secondary battery 703. As a result of this configuration timepiece circuit 702 will remain stopped for a significant period of time until secondary battery 703 is charged to a voltage at which timepiece circuit 702 can operate. This is a time consuming process and a major drawback to known timepieces with solar batteries.
The prior art has also developed charge control circuits for preventing the overcharging of the secondary battery. Such prior art includes U.S. Pat. Nos. 3,731,471 and 3,979,656. However, these references fail to consider the problem of the voltage across the secondary battery being too low to operate the circuitry.
Therefore, there are several basic problems with the prior art self-charging electronic timepieces. In particular, there is a need for a self-charging electronic timepiece which begins to operate immediately after exposure to light even when the secondary source is completely discharged.